Thermoplastic polyurethane elastomers (TPUs) have long been known. They are of commercial importance due to their combination of high-grade mechanical properties with the known advantages of cost-effective thermoplastic processability. A wide range of variation in their mechanical properties can be achieved by the use of different chemical synthesis components. A review of TPUs, their properties and applications is given in Kunststoffe [Plastics] 68 (1978), pages 819 to 825, and in Kautschuk, Gummi, Kunststoffe [Natural and Vulcanized Rubber and Plastics] 35 (1982), pages 568 to 584.
TPUs are synthesized from linear polyols, mainly polyester diols or polyether diols, organic diisocyanates and short chain diols (chain extenders). Catalysts may be added to the reaction to speed up the reaction of the components.
The relative amounts of the components may be varied over a wide range of molar ratios in order to adjust the properties. Molar ratios of polyols to chain extenders from 1:1 to 1:12 have been reported. These result in products with hardness values ranging from 80 Shore A to 75 Shore D.
TPUs can either be produced in stages (prepolymer method) or by the simultaneous reaction of all the components in one step (one shot). In the former, a prepolymer formed from the polyol and diisocyanate is first formed and then reacted with the chain extender.
TPUs may be produced continuously or batch-wise. The best-known industrial production processes are the so-called belt process and the extruder process. By mixing them with other polymers, the favorable properties of TPUs can be modified. In particular, the admixture of graft rubbers improves the low temperature properties of TPUS. A TPU/ABS mixture is described in U.S. Pat. No. 4,317,890. The molding compositions obtained, however, exhibit unsatisfactory impact strength at low temperatures and are not free from segregation phenomena, the so-called "mother of pearl" effect. The use of TPUs of low density (EP-A 0,152,049) or the addition of auxiliary processing agents (polyacrylates in U.S. Pat. No. 4,179,479, styrene-MSA copolymers in EP-A 0,443,432) does not impart a significant improvement to the compositions, which are characterized, in particular, by their modest low-temperature impact-strength.
Another route is described in DE-OS 2,854,409 (U.S. Pat. No. 4,342,847). The TPU-polymer mixtures are produced in an extruder reaction process by adding all the TPU raw materials to the pre-produced polymer, which has previously been melted in the extruder. The mixtures produced in this manner exhibit improved behavior at low temperatures. Their disadvantages, however, are the high melting temperatures required for the copolymer, which result in thermal degradation of the product, or the friction effects which occur at lower melting temperatures, which likewise result in degradation of the product. Moreover, during a continuous process at the low temperatures which are customary for a mixed phase of a TPU reaction (100.degree. to 200.degree. C.), extreme variations occur in the viscosity of the copolymer melt, which cause pressure fluctuations when metering in the TPU monomer. In a continuous process, this does not permit the TPU components to be metered in uniformly over an extended period, which can lead to inhomogeneities in the product or even to a stoppage of the continuous process.
Therefore, no success has hitherto been achieved in producing TPU-copolymer mixtures having good mechanical and elastic properties, particularly at low temperatures, in a continuous process over an extended period without interruptions.